This invention relates to rotatable fluid displacing devices of the meshing gear type and is an improvement over such a device as disclosed in prior German application No. P 27 18 148.2-15 owned by the assignee of the present application.
A fluid displacing device of the foregoing type is operative either as a pump or a motor and is particularly useful as part of a hydrostatic steering system for automotive vehicles. When applied to a hydrostatic steering system, the fluid displacing device is generally attached to the steering control valve housing by means of threaded fasteners extending from an end cover assembled onto an axial end of the valve housing. According to the structural arrangement disclosed for example in applicant's prior German application No. P 28 10 902.6-21, longitudinal flow passages are formed in the valve housing by bores into which the fasteners extend with radial flow clearance. During operation of the fluid displacing device, about half of the flow passages conduct flow to or from the volumetrically decreasing fluid displacing pockets of the fluid displacing device while the other flow passages conduct flow to or from the volumetrically increasing fluid displacing pockets. Further, the valve housing is provided with additional passages for check valves and to conduct fluid in connection with the use of the fluid displacing device as a flow metering motor in a hydrostatic steering system. By utilizing flow passages in the housing surrounding the shanks of the fasteners interconnecting the housing with the fluid displacing device, more room is available within the housing for the formation of the additional passages required in connection with various installations. The fasteners according to prior art arrangements always extend through the radially outer ring gear between the gaps of its internal teeth. A spacer disposed between the fluid displacing device and the valve housing seals the fluid displacing pockets and is provided with radial flow openings through which operating fluid is transferred between the tooth gaps of the outer ring gear and the longitudinal passages in the valve housing surrounding the shank of the fasteners. Such flow paths offer a relatively high flow resistance since the operating fluid undergoes a change in direction twice; first when being conducted in an axial direction from the tooth gaps to a radial direction within the flow passages in the spacer; and second from the radial direction of flow within the spacer to the axially extending passages within the valve housing. The manufacture of the spacer through which the flow passages extend is quite costly since it requires special machining. Where the spacer is produced by a sintering process, a special tool is required to form the flow passages. Further, since the spacer must be dimensioned for a tight fit in its installation, plastic or copper inserts are utilized making production of the spacer even more expensive.
It is therefore an important object of the present invention to provide an assembly of a fluid displacing device and steering control valve of the aforementioned type having a sealing spacer between the fluid displacing device and the valve housing through which flow may be conducted without directional change in order to reduce frictional flow resistance and to enable production of the spacer in a less costly manner.